The study objective is to determine whether 400 mg/kg/day oral L-serine is sufficiently safe and tolerable and possesses an adequately verified neuroprotective mechanism to justify further investigation in a future phase III trial in hereditar sensory and autonomic neuropathy type 1 (HSAN1). Hereditary sensory and autonomic neuropathy type I (HSAN1) is a progressive and debilitating illness for which currently no treatment exists. We recently identified two novel deoxysphingolipids (dSL) that accumulate in plasma of HSAN1 patients and mutant transgenic HSAN1 mice. The disease is caused by missense mutations in the SPTLC1 gene encoding a subunit of the enzyme serine palmitoyltransferase (SPT). In normal circumstances the SPT enzyme catalyzes the reaction of palmitoyl-CoA with serine to form sphinganine. The two newly identified dSL, deoxysphinganine and deoxymethylsphinganine, arise from condensation of palmitoyl-CoA with alanine and glycine respectively, suggesting that HSAN1 mutations alter amino acid selectivity of SPT. In support of this hypothesis we have shown that levels of dSL in humans and mice can be lowered by supplementation with the enzyme's normal substrate, serine. In this randomized, double-blind, placebo-controlled study we will enroll 20 research participants with HSAN1 with 10 subjects assigned to L-serine (400mg/kg/d) and 10 assigned to placebo who are each treated for 1 year, followed by cross-over to L-serine by all participants for one additional year. Pharmacokinetic studies will occur at the start of the first and second year allowing us to complete two PK series, with placebo at baseline providing information on diurnal patterns of dSL levels and an active arm at 1 year providing information on PK of L-serine on top of a stable background. Further, we will determine the impact of long-term reduction of dSL levels relative to neuropathy measures. We will study an existing neurological rating scale of sensory neuropathy, neurophysiological measures and intraepidermal nerve fiber density and assess their variability and sensitivity to L-serine intervention. Importantly it will let us decide on th best outcome measure for future Phase III trials.